Hi,
For these ones, which sentence is the best one to use? Or do they all mean the same thing? So it doesn't really matter?
1. Slot time is the amount of time after which data is retransmitted after a collision has occurred. Sounds fine. But you need to make this 'in which' type of change to every sentence. I hope I have understood your meaning correctly, but anyway, the way you wrote it with 'that' is ungrammatical.
2. Slot time is the amount of time that data is retransmitted after a collision occurs Sounds fine. I think this is the best, clearest and most direct.
3. Slot time is the amount of time that data is retransmitted after a collision occurred. (Is this sentence incorrect with occurred? If it is correct, is this the same as #2 except this one I'm say it has occurred? Where as #2 I'm saying when it 'occurs' ?) No, don't say this.
4. Slot time is the amount of time that data has been retransmitted after a collision has occurred. OK if you are just talking about the past, but I think what you want is to talk generally.
5. Slot time is the amount of time that data has been retransmitted after a collision occurs. No, don't say this.
6. Slot time is the amount of time that data has been retransmitted after a collision occurred. No, don't say this.
Best wishes, Clive